1. Field of the Invention
The present invention relates to the field of bicycles and more particularly to off-pavement or dirt bicycles and frames therefor.
2. Discussion of the Prior Art
Oweing in large part to the current great popularity of off-road or "dirt" motorcycles for adults and older teenagers, there has arisen a corresponding popularity with youngsters for bicycles which duplicate or simulate many features of such motorcycles. These special bicycles, which are particularly designed for rough, off-pavement use on vacant lots and hillsides, enable those too young to use motorcycles, or those whose parents either cannot afford expensive motorcycles or consider motorcycles too dangerous for youngsters, to enjoy at least some of the thrills and excitement of dirt cycling and motocross racing competition.
In addition to being safer for youngsters than motorcycles and much less costly, these so-called dirt bicycles have further advantages in that they can be ridden on streets like conventional bicycles and can be used in urban and suburban areas where dirt motorcycling is prohibited because of noise, lack of space, etc. Thus, even though a youngster may own or have access to a dirt motorcycle, for more frequent and convenient use he may also want a dirt bicycle.
While conventional bicycles are generally subject to frequent hard abuse, dirt bicycles are subjected to even more severe hard use, for example, when the rider leaps it into the air, jumps ruts and ditches with it and rides it over rock strewn ground, not to mention frequently dumping it when spills are taken.
Therefore, dirt bicycles in particular must be constructed in an extremely rugged manner to assure a reasonable usage life. Nevertheless, they must be made reasonably light weight so they may be easily handled by fairly young children, and they must be priced competitively with conventional bicycles, as most parents are usually unable or unwilling to pay much more than conventional bicycle prices just so their children can play at motorcycling. Still further, although the dirt bicycles must be ruggedly built, and are preferably constructed to look at least somewhat like a motorcycle, they must still have a pleasing appearance--that is, they cannot be constructed having all sorts of unsightly braces and trusses to give them additional strength.
As a result of such off-pavement rough usage, many construction techniques used for regular bicycles are unsuitable for dirt bicycles. At least two portions of the bicycle frames are particularly vulnerable to damage or failure. One of these regions is that of the crank hub. Ordinarily, this hub is positioned at a lower corner of a generally triangular frame, and usually tubular frame members are welded directly to the hub or to hub sockets so that the hubs are also a major frame joint. Because of great stress concentration at the hub region and non-continuity of the frame in this region, such type frames when subject to severe stresses, often crack or break at the hub attachment welds or require unsightly gussetting in this area. In the event of complete frame breakage at this point, the rider may be severely injured. In addition, because the hub is at a lowest point of the frame--that is, normally closer to the ground than adjacent frame portions--it is susceptible to easy damage by large rocks or high curbs. Also, in off-pavement use, this type hub may be frequently dragged in the dirt, with the result that sand and grit will get into the crank bearings and cause bearing failure.
The second region of common frame failure is the front wheel fork. Whereas the rear wheel support is fairly rugged, the cantilevered front wheel fork is comparatively fragile and yet it is subjected to very high bending stresses in the swivel attachment region, particularly when the bicycle is jumped and lands on the front wheel. Bending of the fork causes the front wheel to be misaligned and may cause spills; complete or partial breaking of the fork in use can result in severe rider injury.
Heretofore, in most cases, attempts to increase the strength of these critical frame portions for dirt bicycles have resulted in excessively heavy, expensive and/or unsightly bicycles, and frame improvements, particularly in heavy-duty frames for dirt-type bicycles, are necessary.